A data storage device based on the atomic force microscope (AFM) is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes operate in parallel, with each probe scanning, during operation, an associated field of the storage medium. The storage medium comprises a polymer layer. The tips, which each have a diameter of between 20 nanometers (nm) to 40 nm, are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying forces to the probes so that the tips of the probes can touch the surface of the polymer layer. For this purpose, the probes comprise cantilevers, which carry the tips on their end sections. Bits are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes while they are moved across the surface of the polymer layer during operation of the device in read/write mode.
Indentation marks are formed on the polymer layer by thermomechanical recording. This is achieved by heating the tip of a respective probe operated in contact mode with respect to the polymer layer. Heating of the tip is achieved via a heater dedicated to the writing/formation of the indentation marks. The polymer layer softens locally where it is contacted by the heated tip. The result is an indentation being produced on the layer that has, for example, a nanoscale diameter comparable to the diameter of the tip used in its formation.
Reading is also accomplished by a thermomechanical concept. The probe is heated using a heater dedicated to the process of reading/sensing the indentation marks. In this case, the probe is heated but not so as to cause heating of its associated tip. That is, the heating temperature is not enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium changes when the probe is moving in an indentation as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence its electrical resistance changes. This change of electrical resistance is then measured and serves as the measuring signal.
Presently, polymer layers for use in storage devices described above are prepared by spin coating a polymer of choice onto a substrate such as, for example, a silicon wafer. The surface roughness of a polymer layer that is produced with this method is on the order of, for example, 1 to 2 nm when measured over a typical bit dimension length scale.
As described above, information is encoded into the polymer layer in the form of indentation marks and non-indentation marks. Taking into account the surface roughness values typically obtained with spin coating, it is desirable to achieve a signal-to-noise ratio (SNR) of >10 dB over the lifetime specification of the storage device for sensing the information. In order to achieve such a sensing margin, each of the indentations should typically be formed with a depth on the order of, for example, 10 nm. Since the lateral dimensions of the indentations are on the same order of magnitude as their depth, it is inevitable that the recording density of the storage device is accordingly limited.
Accordingly, it is desirable to provide a method of producing a data storage medium that, when incorporated in a data storage device, allows for an increased recording density to be attained by such a device compared to a data storage medium produced using previously-proposed methods such as, for example, spin coating.